This invention relates to the treatment of hot geothermal fluids containing hydrogen sulfide, and more particularly to the oxidation of hydrogen sulfide present in a two-phase geothermal fluid in order to prevent scale deposition therefrom.
As conventional hydrocarbon fuel reserves have been depleted, a worldwide emphasis has been placed on developing alternative energy resources, such as geothermal energy. Geothermal energy is generated by producing steam, hot water or hot aqueous brines from a subterranean geothermal reservoir and utilizing the produced fluid in conjunction with surface equipment, such as turbines and heat exchangers, to derive useful energy therefrom.
Most hot aqueous geothermal brines which are produced from subterranean reservoirs contain substantial quantities of dissolved heavy and/or transition metal ions and also hydrogen sulfide. As the geothermal brine is produced via wells communicating with a subterranean geothermal reservoir, the pressure is reduced and the brine begins to boil or flash to form a gaseous phase which causes the temperature of the fluid to drop. Concomitant with this pressure and temperature drop, carbon dioxide evolves from the brine thereby shifting the equilibria existing therein and causing the disassociation of divalent sulfide ions from hydrogen sulfide. These divalent sulfide ions react with the heavy and/or transition metal ions to form heavy and/or transition metal sulfide precipitates which cause fouling of the process piping and equipment.
In view of this, several techniques for reducing the formation of insoluble metal sulfide precipitates in geothermal brine and the resultant scaling have been proposed. One such technique is to oxidize the sulfides present in the brine to more soluble sulfur species. It has been suggested to add oxygen to low salinity brines just prior to entry of the brine into the first surface flash vessel. In this manner, sulfides in the brine would be converted mostly to sulfate with some elemental sulfur being formed.
In a 1974 report on a series of laboratory experiments conducted with a simulated brine, one investigator disclosed that the rate of sulfide conversion to sulfate was very high upon the addition of 6 to 10 moles of oxygen per mole of sulfide present in the simulated brine. The simulated brine did not contain barium and therefore the effect of scaling due to the formation of barium sulfate was not reported.
Another investigator proposed in a 1976 report that the use of lesser amounts of oxygen, i.e., less than the stoichiometric amount of oxygen required to convert the sulfide to sulfate, may result in only a partial oxidation of the sulfide to, for example, elemental sulfur thereby avoiding the formation of sulfate scales, such as barium sulfate. The concept had not been tested in the laboratory or in the field.
However, a 1977 report by the first-mentioned investigator presented the results of additional laboratory tests which established that the hoped-for partial oxidation was not achieved when less than stoichiometric amounts of oxygen, such as 0.5 to 2.0 moles of oxygen per mole of hydrogen sulfide, were injected into the simulated brine. This report disclosed that oxidation of the sulfide was complete at molar ratios of 1.25:1 and 1.5:1 forming mostly sulfate species, such as more than 80 percent sulfates.
Accordingly, the prior art teaches that the partial oxidation of barium-containing geothermal fluids in order to prevent the deposition of sulfide and/or sulfate scales is not possible even at substoichiometric oxygen to hydrogen sulfide molar ratios. A need exists for a method which will oxidize sulfides present in the geothermal fluid so as to prevent sulfide scale deposition but which at the same time does not deposit significant amounts of sulfate scale.
Accordingly, it is an object of the present invention to provide a process for oxidizing sulfides present in a two-phase geothermal fluid in a manner which substantially eliminates any attendant precipitation, scaling and corrosion problems.
Another object of the present invention is to provide an economical and efficient process for oxidizing sulfides present in geothermal brines in order to prevent the deposition of sulfide and/or sulfate scale.
A still further object of the present invention is to provide a process for oxidizing sulfides present in geothermal brines to an intermediate oxidation state less than sulfate in order to substantially eliminate barium sulfate deposition.
These and other objects and advantages of the invention will be apparent from the following detailed description and drawing.